FEEDING PARAMECIUM KNOWN BACTERIA 
workers from entering this field and forced them to be content 
with more or less general methods of food control. 
Meissner ('88) was one of the first to investigate the food habits 
of Protozoa. His experiments dealt with the digestibility of 
the various types of food, such as starch grains, oil drops, and 
particles of albumen. His work is of importance in demonstrat- 
ing that the Protozoa are not essentially unlike the Metazoa in 
their power to utilize food. Moreover, Meissner showed that not 
all types of food are digested with equal ease, and some not at all. 
These findings are of significance in suggesting that variations 
in digestibility may exist in the normal food of these animals 
which are of importance when one is attempting to study their 
behavior. 
Environmental factors other than food have received the 
greater amount of attention of investigators of the Infusoria. 
Of such factors, the nature of the medium has been perhaps too 
much emphasized. That the medium is important, there can 
be no doubt, but if it is of such a nature as to fulfill the osmotic 
requirements of animals living in it, and to furnish the proper 
salt content, it would seem that its importance, as far as Para- 
mecium is concerned, lies in its ability to furnish a proper food 
for the bacteria upon which this animal feeds. Calkins at first 
('02 a) thought of the medium as being of main importance, but 
he soon came to appreciate the relation of bacteria ('02 b). His 
statement that B. subtilis was the chief food of his Paramecia is 
probably not justified. We now know that the variety of bacteria 
living in an ordinary hay infusion is not confined to a single 
group. Moreover, bacteriologists have subdivided the group of 
bacteria known as B. subtilis until it includes a large number of 
forms. Not all of these will support the life of Paramecium, as 
is shown by a comparison of my results With those of Hargitt and 
Fray ('17). Calkins was careful to sterilize his media by heating 
to 90°, but we now know that this temperature is not sufficient 
to kill many spores. No attempt was made to exclude air- 
borne bacteria which he assumed furnished the greater part of the 
bacterial food. He may, therefore, have been using an infusion 
in which deleterious bacteria came to predominate. 
